1. Field of the Invention
The present invention relates to an image projection apparatus such as a liquid crystal projector for magnifying an image formed by means of image display elements (liquid crystal panels) and displaying it on a projection surface such as a screen.
2. Description of the Related Art
Hitherto, image projection apparatuses for magnifying an image formed by means of image display elements (liquid crystal panels) and projecting it on a projection surface includes an illumination system for guiding light from a light source into the image display elements and a projection system for projecting the light from the image display elements onto a screen or the like by focusing the image thereon. FIG. 8 illustrates the configuration of an image projection apparatus using a crossdichroic prism. The configuration includes a light source (lamp) 101; a reflecting mirror 102; lens arrays 103 and 104 having a plurality of lenses; a polarization conversion element 105 for aligning unpolarized light rays in a predetermined polarization direction; a condenser lens 106 for focussing illumination light into a predetermined illumination area; reflecting mirrors 107a and 107b; dichroic mirrors 108 and 109 for separating white light into light of predetermined colors; image display elements 110G, 110B, and 110R; a relay optical system 111 having lenses 111a, 111b, and 111c, and mirrors 111d and 111e, for efficiently transmitting the illumination light to the image display elements 110G, 110B, and 110R over long optical paths; field lenses 112G and 112R for causing the illumination light to correctly enter the image display elements 110G and 110R; a color-combining prism 113 for directing the light from each of the image display elements 110G, 110B, and 110R onto one optical path; and a projection lens 114. Lenses composing the projection lens 114 are held in a lens holding section 115a, and the image display elements 110G, 110B, and 110R and the color-combining prism 113 are held in a holding section 115b. The light source (lamp) 101 that is mounted on the reflecting mirror 102 is held in a lamp holding section 115c, and other optical members are held in a lens barrel (optical element holding means) 115d. 
In recent years, so-called image display elements with microlenses, which are provided with a micro lens for every pixel included therein in order to increase light efficiency, have come into widespread use. Generally, it is often the case that image display elements with microlenses are used when brightness is important and image display elements without microlenses are used when cost is important. In such a case, the same arrangements have been used for the illumination systems, which illuminate the image display elements, for both image display elements with microlenses and without microlenses.
Usually, because the illumination system is set so as to have the highest brightness, it is designed to be adaptable to the arrangement using the image display elements with microlenses, in which brightness is important. Since the microlenses have a function for focussing the light into pixel apertures of the image display elements, it is possible to decrease the amount of light loss at the pixel apertures when light rays incident on the microlenses are more parallel to each other. Thus, it is desirable that the illumination system has a larger F-number. A detailed description will be given with reference to FIG. 9. FIG. 9 illustrates the illumination system from the light source (lamp) 101 to the image display element, in which the mirrors and the dichroic mirrors in the optical system are omitted. Here, the F-number of the illumination system is the F-number of the luminous flux of the illumination light for the image display element. Provided that the F-number of the illumination system is represented by F, the pupil diameter of the illumination system is represented by Q, and the composite focal length of the condenser lens 106 and the field lens 112 is represented by f, the F is given by the following equation:F=f/Q. When a lens array having a predetermined size is used (Q is constant), it is necessary to increase the value of f in order to increase the value of the F-number for the illumination system (in order to darken the illumination system). Here, the pupil diameter Q for the illumination system is twice as large as the radius q of a circumcircle (shown by a dotted line) encompassing light source images I formed on the lens array 104, as shown in FIG. 10.
Another factor for determining the brightness of the illumination system is the polarization conversion element. In the polarization conversion element, the incident light rays are separated into polarization components P and S by a polarization beam splitter, and the polarization component S is reflected by a reflecting surface thereof so that the traveling direction of the polarization component S is made to be the same as that of the polarization component P. Provision of a ½ wave plate on an exit plane for the polarization component P causes the polarization component P to be converted into the polarization component S. Because one incident optical path is divided into two optical paths in the polarization conversion element as described above, the incident light rays must be spatially discrete luminous fluxes corresponding to incident apertures for the polarization conversion element, which are arranged in a stripe pattern. This is realized by the lens array 103 disposed at the incident side of the polarization conversion element 105 (referring to FIG. 9). The lens array has a configuration in which a plurality of lenses is two-dimensionally arranged. The light incident on the lens array is focussed on the optical axis of each lens composing the lens array. Setting the focal point of the light in close proximity to the polarization conversion element makes it possible to cause spatially discrete luminous fluxes corresponding to the incident apertures to be incident on the polarization conversion element. More light rays incident on the incident apertures A of the polarization conversion element allow the light efficiency in the polarization conversion element to increase, as shown in FIG. 11. To this end, it is necessary to reduce the area where the light is focussed at the lens array 103 (the area of the light source images I). Since the light source (lamp) 101 generally has a finite size, the light collimated by the reflecting mirror 102 is incident on the lens array 103 at a predetermined angle. Accordingly, the smaller the focal length of each lens composing the lens array 103, the smaller the area where the light is focussed, thus increasing the light efficiency. Provided that each of the lens apertures for the lenses composing the lens array 103 has a constant size, this corresponds to decreasing the F-numbers of the lenses composing the lens array 103.
The lens arrays 103 and 104 have a function of forming a uniform illumination area on the image display element. In order for the lens arrays to perform this function, the following condition must be met:P/f≈p/f2 where P represents the size of the uniform illumination area, f2 represents the composite focal length of the lens arrays, and p represents the size of each of the lenses composing the lens arrays, as shown in FIG. 9. Accordingly, the following condition is met:F′=f2/p≈f/P where F′ represents the F-number of a lens array unit. Because the size P of the illumination area is predetermined when the image display element has a predetermined size, it is necessary to decrease the value of f in order to decrease the value of the F-number F. This condition is incompatible with the condition described above. Thus, in practical optimization, the value of f is determined so as to obtain the maximum light efficiency by balancing the light efficiency in the microlenses with the light efficiency in the polarization conversion element.
In the image display element without microlenses, because high apparatus efficiency is obtained when the polarization conversion element has high efficiency, the illumination system optimized for the image display element with microlenses, as described above, is not optimal for the apparatus using the image display element without microlenses. Thus, this has caused a substantial decrease in brightness, compared with the image display element with microlenses.